1. Field
This invention generally pertains to the repair of cracks in and/or the repair/replacement of crack-susceptible materials in sealed penetrations in a pressure vessel and more particularly to the repair of bottom-mounted instrumentation nozzles in nuclear reactors.
2. Related Art
Cracks in alloy 600 material are commonly experienced in operating nuclear power plants and are prevalent when the alloy 600 materials are directly exposed to primary coolant. Alloy 600 material cracking has forced the industry to develop a variety of repair and replacement options that mitigate these conditions. Alloy 600 bottom mounted instrumentation nozzles and their associated installation welds (J-welds) are in direct contact with the primary coolant water and are known to be crack susceptible.
Because bottom mounted instrumentation nozzles are located at the bottom of the reactor pressure vessel, bottom mounted instrumentation nozzle repair or replacement poses considerable challenges. These challenges include high radiation levels, restricted access to the J-weld (i.e., access from inside the vessel) and restricted access to the bottom mounted instrumentation-to-in-core instrumentation weld in the in-core instrumentation pit (i.e., access from outside and beneath the vessel). Because bottom-mounted instrumentation nozzle repairs must be accomplished with the reactor pressure vessel internals removed, the reactor pressure vessel must remain filled with water during any repair efforts; this water level serves to keep the highly radioactive reactor pressure vessel internals, which are stored adjacent to the reactor pressure vessel in the refueling pool, under water. Bottom mounted instrumentation nozzles are typically 30 feet or more beneath the water surface. As a result, any work from within the vessel requires remote tooling designed specifically for this underwater environment. Any repair that breaches the reactor pressure vessel pressure boundary must include provisions to mitigate loss of the primary water inventory since any loss of primary coolant inventory will directly affect plant safety. Welded repairs to the bottom mounted instrumentation nozzles/J-welds must deliver code acceptable weld quality unaffected by the presence of water. Other welding challenges include the need for temperbead welding (where applicable) to avoid requirements for post weld heat treatment when welding directly to the low alloy steel reactor pressure vessel or welding on the cladding within one-eighth inch (0.32 cm) of the low alloy steel reactor pressure vessel shell, the need for welding to the existing alloy 600 buttering (immediately adjacent to the J-weld) or the need for welding to the cladding (cladding is a nonstructural weld that requires additional evaluation/testing in order to be considered structural). Thus, the need exists for a method for the removal of the existing bottom mounted instrumentation nozzle and all or portions of the J-weld, and the welded installation of a replacement bottom mounted instrumentation nozzle or plug constructed from a crack-resistant nickel alloy.